U. PITTSBURGH (US) — Thousands of novel, undiscovered viruses, some of which could affect human health, are believed to be hiding in raw sewage, new research shows.

There are roughly 1.8 million species of organisms on the planet, and each one is host to untold numbers of unique viruses, but only about 3,000 have been identified to date.

To explore this diversity and to better characterize the unknown viruses, researchers developed techniques to look for genetic signatures of novel viruses in various locations in North America, Europe, and Africa and found signatures from 234 known viruses, representing 26 different families of viruses in raw sewage, making it home to the most diverse array of viruses yet found.

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“What was surprising was that the vast majority of viruses we found were viruses that had not been detected or described before,” says Roger Hendrix, professor of biological sciences at University of Pittsburgh.

The viruses that were already known included human pathogens like Human papillomavirus and norovirus, which causes diarrhea. Also present were several viruses belonging to those familiar denizens of sewers everywhere: rodents and cockroaches.

Bacteria are also present in sewage, so it was not surprising that the viruses that prey on bacteria dominated the known genetic signatures. Finally, a large number of the known viruses found in raw sewage came from plants, probably owing to the fact that humans eat plants, and plant viruses outnumber other types of viruses in human stool.

Published in the online journal mBio, the study was also the first attempt to look at all the viruses in the population. Other studies have focused on bacteria, or certain types of viruses. The researchers also developed new computational tools to analyze the data. The approach, called metagenomics, had been done before, but not with raw sewage.

The main application of this new technology will be to discover new viruses and to study gene exchange among viruses.

“The big question we’re interested in is, ‘Where do emerging viruses come from?'” says Hendrix. The team’s hypothesis is that new viruses emerge, in large part, through gene exchange. But before research on gene exchange can begin in earnest, large numbers of viruses need to be studied.

“First you have to see the forest before you can pick out a particular tree to work on,” says co-author James Pipas, professor of biological sciences. “If gene exchange is occurring among viruses, then we want to know where those genes are coming from, and if we only know about a small percentage of the viruses that exist, then we’re missing most of the forest.”

Researchers from Washington University in St. Louis and the University of Barcelona contributed to the study.
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